Plant pests are a major factor in the loss of the world's commercially important agricultural crops resulting both in economic hardship to farmers and nutritional deprivation for local populations in many parts of the world. Broad spectrum chemical pesticides have been used extensively to control or eradicate pests of agricultural importance. There is, however, substantial interest in developing effective alternative pesticides.
Control of various pests through the use of biological molecules has been possible in only a limited number of cases. The best known examples of biological molecules with pesticidal uses are the .delta.-endotoxins from Bacillus thuringiensis (Bt), which is a gram-positive spore forming microorganism. Varieties of Bt are known that produce more than 25 different but related .delta.-endotoxins. Bt strains produce .delta.-endotoxins during sporulation.
The majority of .delta.-endotoxins made by Bt are toxic to larvae of certain insects in the orders Lepidoptera, Diptera and Coleoptera. Some of these .delta.-endotoxins have useful insecticidal activities against different insect pests. However, use of the .delta.-endotoxins is limited because they are active against only a very few of the many insect pests.
The limited specificity of the Bt endotoxins is dependent, at least in part, on both the activation of the toxin in the insect gut (Haider, M. Z. et al., 1986, Eur. J. Biochem. 156:531-540) and its ability to bind to specific receptors present on the insect's midgut epithelial cells (Hofmann, C. P. et al., 1988, PNAS 85:7844-7848). Among the factors which prevent activity of a particular .delta.-endotoxin against a specific insect is the lack of appropriate receptors in the insect gut or lack of affinity of the .delta.-endotoxin for the receptors which may be present, thus resulting in no binding of the .delta.-endotoxin to the brush border membranes. Therefore, the ability to control a specific insect pest using .delta.-endotoxins at present depends on the ability to find an appropriate .delta.-endotoxin with the desired range of activity. In many cases, no such .delta.-endotoxin is known, and it is not certain that one even exists.
Plants also routinely become infected by fungi and bacteria, and many microbial species have evolved to utilize the different niches provided by the growing plant. Some phytopathogens have evolved to infect foliar surfaces and are spread through the air, from plant-to-plant contact or by various vectors, whereas other phytopathogens are soil-borne and preferentially infect roots and newly germinated seedlings. In addition to infection by fungi and bacteria, many plant diseases are caused by nematodes which are soil-borne and infect roots, typically causing serious damage when the same crop species is cultivated for successive years on the same area of ground.
The severity of the destructive process of disease depends on the aggressiveness of the phytopathogen and the response of the host, and one aim of most plant breeding programs is to increase the resistance of host plants to disease. Novel gene sources and combinations developed for resistance to disease have typically only had a limited period of successful use in many crop-pathogen systems due to the rapid evolution of phytopathogens to overcome resistance genes. In addition, there are several documented cases of the evolution of fungal strains which are resistant to particular fungicides, such as powdery mildew, wheat mildew, Botrytis, Pyrenophora, Pseudocercosporella and Mycosphaerella fijiensis (Jones and Clifford; Cereal Diseases, John Wiley, 1983).
It is apparent, therefore, that scientists must constantly be in search of new methods with which to protect crops against plant pests. It has been found in the present invention a novel class of proteins which can be used to control plant pests.
Programmed cell death is a process whereby developmental or environmental stimuli activate a genetic program that culminate in the death of the cell (Jacobson, M. D. et al., 1997,Cell 88: 347-354). This genetic potential exists in most, if not all, multicellular organisms. In the case of invertebrates, programmed cell death appears to play a dual role by being an integral part of both the insect development process and a response mechanism to infections particularly of viral nature (Clem, R. J. et al., 1991,Science 254: 1388-1390). Programmed cell death appears to be executed in several different manners leading to either apoptosis, atrophy or differentiation. Apoptosis is one of the best characterized types of programmed cell death encompassing cytological changes including membrane-bound apoptotic bodies and cytoplasmic blebbing as well as molecular changes such as endonucleolysis typified by the generation of oligosomal length fragments (Vaux, D. L and Strasser, A., 1996, PNAS 93:2239-2244). Although the overall apoptotic phenomenology is rather conserved among the different organisms, it is interesting to point out that, for many insect cells, cytoplasmic vacuolization and swelling rather than condensation seem to be the cytological features associated with apoptotic processes (Bowen, I. D., et al., 1996, Micros. Res. Techniq.34:202-217). The present invention provides a novel class of proteins which induce programmed cell death and exert a pesticidal effect.